In the manufacture of a magnetic recording medium such as magnetic tape, a magnetic paint is coated on a nonmagnetic support such as a nonmagnetic polymeric film and the magnetic field orientation treatment is performed in order to improve magnetic characteristics of magnetic substance of a magnetic layer in a particular direction. According to a conventional method for manufacturing a magnetic recording medium, needle-like magnetic particles are dispersed in a binder to prepare a magnetic paint. The resultant magnetic paint is coated on a nonmagnetic support to a predetermined thickness. While the magnetic particles in the coating film are still mobile, that is, while the coating film is still not dry, the film is passed through a magnetic field so that the magnetic particles are oriented along the direction of the magnetic field, thus completing the magnetic orientation treatment. By performing the orientation treatment, the squareness ratio (Rs) of the magnetic recording medium is increased to improve sensitivity.
With recent demand for higher recording density, a higher S/N ratio is required. For this reason, finer magnetic particles are desired. As a result, when such fine magnetic particles are used, it is difficult to achieve sufficient orientation with the conventional magnetic field orientation treatment. When the size of the magnetic particles decreases, the magnetic moment of the individual magnetic particles is also decreased. As a result of this, the torque which acts on the particle is weakened to interfere with orientation. Furthermore, when the magnetic particles become smaller in size, the specific surface area increases. Therefore, the viscosity resistance against the movement of the magnetic particles in the magnetic paint increases to render orientation difficult.
Although fine alloy powders which have great magnetic moments are also recently used for manufacturing magnetic recording media, they tend to easily cohere magnetically to significantly impair orientation. Satisfactory orientation may not be expected with such fine alloy powders when a conventional orientation treatment is performed.
A permanent magnet or a DC electromagnet is used in the conventional orientation treatment. According to such a method, the degree of orientation is not much improved even if an applied magnetic field is intensified in an attempt to improve the degree of orientation of the magnetic particles in the magnetic layer. When the magnetic field is intensified exceeding a predetermined level, the smoothness of the surface of the magnetic film is impaired. Various methods have been proposed in order to improve such a orientation treatment method. For example, Japanese Patent publication No. 49-30722 proposes an orientation treatment method in which an auxiliary magnetic field generator for superposing an AC auxiliary magnetic field on the main magnetic field is arranged in the vicinity of a main orientation device using a permanent magnet or a DC electromagnet. Japanese Patent Publication No. 54-98205 proposes a method for facilitating orientation of magnetic particles by superposing an AC magnetic field or mechanical vibrations perpendicularly to the direction of the main orientation of a DC magnetic field. However, these methods do not increase the rotational torque for moving the magnetic particles but only serve to help the movement of the magnetic particles in a main orienting magnetic field.
According to the conventional methods including the well-known methods described above, the intensity of a magnetic field available is 2 KOe and is about 3 KOe at maximum. Even if fine particles are used as magnetic particles, the intensity of a magnetic field which can supply a sufficient rotational torque is as high as about 5 kOe. A magnetic field of a still higher intensity is required if alloy-type ultra fine particles are used as magnetic particles. Even if magnetic particles of oxide type are used, a magnetic field of a higher intensity is required to disperse the magnetic particles if the viscosity of a viscous medium such as a binder to be used is high. Therefore, with the conventional method, the intensity of the magnetic field for providing a rotational torque necessary to satisfactorily orient the magnetic particles is not sufficient. Even if the intensity of a magnetic field is to be made higher, the magnetic material such as a permanent magnet used is limited and a magnetic field of about 3 kOe or higher cannot be generated.
The conventional method is subject to another problem: a magnetic field application time to provide a rotational torque necessary to rotate the magnetic particles is short. In the conventional method, if the speed of the magnetic tape during the orientation treatment is 100 m/min and the gap for generating magnetic flux is 1 cm, the magnetic field application time is as short as 6 msec. In contrast to this, the magnetic field application time required to complete satisfactory orientation of the magnetic particles is about 1,000 msec if the intensity of the magnetic field is about 2 kOe and the viscosity of the viscous medium is low, and is about 5,000 msec if the intensity of the magnetic field is about 2 kOe and the viscosity of the viscous medium is high. Thus, the magnetic field application time is too short. Even if an attempt is made to elongate the magnetic field application time, it is almost impossible to do so in principle except for making the gap passing speed of the magnetic tape very slow since the magnetic flux from the narrow gap in the magnetic circuit is utilized. If the speed of the magnetic tape during orientation is made very slow for this purpose, the productivity of the medium is lowered to make the manufacture thereof impractical. Thus, the magnetic field application time in the conventional method is far too short for satisfactorily orientating the magnetic particles, resulting in quite unsatisfactory orientation of the magnetic tape.